Digital Joinery, Part 2 – The Board Corner Joint

In the post, Digital Joinery Design Automation, I introduced a project I’m working on to implement all 50 of the “50 Digital Joints” as features in the Onshape CAD system using its FeatureScript language. The idea is simple. If I want to join two (or maybe more) parts with one of the 50 joints, I’d like to model the 3D bodies, simply overlap them where I want the joint, click a “digital joint” feature button, tweak a few parameters, and have the joint geometry generated for me automatically. Simple idea; slightly more complicated implementation.

In the Frame Corner Joint post, I introduced my first attempt at creating a joint whose purpose is to join to long, slender bodies at a (typically 90 degree) corner. In this post, I’ll talk a bit about my implementation of the “Board Corner Joint.” The Board Corner Joint (shown below) is designed to join two large flat bodies at a 90 degree corner. Unlike the Frame Corner Joint, the Board Corner Joint enforces the 90 degree joining constraint for ease of manufacturing (the majority of CNC routing is 3-axis, so the tool axis remains perpendicular to the workpiece). In this post, I’ll go into a little more detail about how the feature itself works, but you can also read the documentation if you’re so inclined.

10 Board Corner Joints from the original 50 Digital Joints collection

So, how does it work?

To start, you’ll want to add my Board Corner Joint FeatureScript to your part studio by following these instructions. Once you’ve added the FeatureScript it will show up as a little icon with the letters “BC” on it.

Custom FeatureScripts show up as icons with the letter(s) of the first word(s) inside a square.

To use the Board Corner Joint, simply draw two bodies representing boards you’d like to join at 90 degrees and overlap them at the corner where you’d like the joint to be generated. I’ve drawn two bodies that are each 3/4 in. thick, 3 inches wide, and 6 inches long.

The basis for the Board Corner Joint is two flat bodies meeting at 90 degrees, overlapped with the edge of one flush to the outer large face of the other

Now, click the “BC” icon that was placed in your toolbar when you added the custom FeatureScript. You’ll get a dialog box asking you to select the type of joint you want, the two parts to join, and some other geometric parameters.

The interface for creating your custom Board Corner Joint.

Selecting the two parts created earlier with the parameters in the figure above gives us the following result:

A Board Corner Joint with three simple finger tenons, 1/4 inch diameter overcut, and no additional allowance.

Allowances make manufacturing easier

A nice feature of this, um, feature, is the ability to add allowances to the joint to make manufacturing (and the subsequent fitting together of parts) easier. By clicking the box to add an allowance, you can add spacing between the parallel faces of the finger tenons. The allowance will be added to faces perpendicular to the long edge of the joint.

In the image above, we can see that the allowance is only added to the “sides” of the tenons, creating a 0.03in gap between the fingers of two parts. This allowance will influence how tightly or loosely the joint fits together. We may also want to adjust how deeply the boards can be pressed together. For example, we might want the fingers to be proud of the outer face on one or both sides of the joint. To do that, we can select the “Apply to mating faces on tenon” and/or “Apply to mating faces on mortise” options.

So, as you can see above, you can add allowances to offset the faces of the finger tenons in all directions in order to adjust the fit of the joint. Below are some images of a few completed Board Corner Joints machined in poplar and walnut. For reference, I used a 0.01″ allowance (and applied it to all the mating faces) and the joints go together pretty tightly. As always, your mileage may vary.

I’ve only scratched the surface of this joint feature. Apart from the simple finger tenons, you can create seven other types of joints (and, more are on the way!). So, if you like fabricating things using CNC routing and you find yourself repeatedly generating right angle corner joinery, give this FeatureScript (and Onshape!) a whirl and let me know how it goes. I’m sure there are lots of ways to improve this feature, but I won’t know until folks try it out and send me feedback.

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